Flat-panel display devices are widely used in conjunction with computing devices, in portable electronic devices, and for entertainment devices such as televisions. Such displays typically employ a plurality of pixels distributed over a substrate in a display area to display images. Each pixel incorporates several, differently colored light-emitting elements commonly referred to as sub-pixels, typically emitting red, green, and blue light, to represent each image element. As used herein, pixels and sub-pixels are not distinguished and refer to a single light-emitting element. A variety of flat-panel display technologies are known, for example plasma displays, liquid crystal displays, and light-emitting diode (LED) displays including organic light-emitting diode (OLED) displays.
Light emitting diodes (LEDs) incorporating thin films of light-emitting materials forming light-emitting elements have many advantages in a flat-panel display device and are useful in optical systems. Color displays that include an array of organic LED light-emitting elements have been demonstrated. Alternatively, inorganic materials can be employed and can include phosphorescent crystals or quantum dots in a polycrystalline semiconductor matrix. Other thin films of organic or inorganic materials can also be employed to control charge injection, transport, or blocking to the light-emitting-thin-film materials, and are known in the art. The materials are placed upon a substrate between electrodes, with an encapsulating cover layer or plate. Light is emitted from a pixel when current passes through the light-emitting material. The frequency of the emitted light is dependent on the nature of the material used. In such a display, light can be emitted through the substrate (a bottom emitter) or through the encapsulating cover (a top emitter), or both.
LED devices can include a patterned light-emissive layer wherein different materials are employed in the pattern to emit different colors of light when current passes through the materials. Alternatively, one can employ a single emissive layer, for example, a white-light emitter, together with color filters for forming a full-color display. It is also known to employ a white sub-pixel that does not include a color filter. One design described in the prior art includes an unpatterned white emitter together with a four-color pixel including red, green, and blue color filters and sub-pixels and an unfiltered white sub-pixel to improve the efficiency of the device.
Two different methods for controlling the pixels in a flat-panel display device are generally known: active-matrix control and passive-matrix control. In a passive-matrix device, the substrate does not include any active electronic elements (e.g. transistors). An array of row electrodes and an orthogonal array of column electrodes in a separate layer are formed over the substrate; the intersections between the row and column electrodes defining the electrodes of a light-emitting diode. Passive-matrix devices are controlled by the sequential activation of, for example, row electrodes while electrodes connected to each column of pixels in an array are provided with respective analog data values. When the row electrode is activated, each column in the row of pixels is driven to a luminance corresponding to the data value on the associated column electrode. The process is sequentially repeated for each row in the pixel array.
External driver chips sequentially supply current to each row (or column) while the orthogonal column (or row) supplies a suitable voltage to illuminate each light-emitting diode in the row (or column). Therefore, a passive-matrix design employs 2n connections to produce n2 separately controllable light-emitting elements. However, a passive-matrix drive device is limited in the number of rows (or columns) that can be included in the device since the sequential nature of the row (or column) driving creates flicker. If too many rows are included, the flicker can become perceptible. Typically, passive-matrix devices are limited to about 100 lines, far fewer than is found in contemporary large-panel displays, for example such as high-definition televisions that have over 1,000 lines and are therefore unsuitable for passive-matrix control. Moreover, the currents necessary to drive an entire row (or column) in a passive-matrix display can be problematic and limits the physical size of a passive-matrix display. Furthermore, the external row and column driver chips for both passive- and active-matrix displays are expensive.
In an active-matrix device, a data value is likewise applied to every column electrode in an array and a select signal associated with a row activated to deposit the data values in a storage element associated with each pixel in the array. Again, the process is sequentially repeated for each row. An important distinguishing characteristic of the active-matrix devices is that the data values are stored with each pixel, thereby enabling the pixel to emit light even when the select signal for that pixel is inactive. In both passive- and active-matrix cases, signal lines form a two-dimensional matrix of vertical and horizontal wires, each driven by external drivers (see, e.g. U.S. Pat. No. 6,232,946). Alternatively, the driving chips can be located on the substrate outside the image display area (see, e.g. U.S. Pat. No. 6,582,980). The wiring for the signals also takes up a considerable area on a substrate, thereby reducing the aperture ratio or increasing the number of metal layers on the substrate and the cost, and is limited in the frequency at which it can operate and the current that can be employed.
In an active-matrix device, active control elements are formed of thin films of semiconductor material, for example amorphous or poly-crystalline silicon, coated over a flat-panel display substrate. Typically, each pixel is controlled by one control element and each control element includes at least one transistor. For example, in a simple active-matrix organic light-emitting (OLED) display, each control element includes two transistors (a select transistor and a power transistor) and one capacitor for storing a charge specifying the luminance of the pixel. Each light-emitting element typically employs an independent control electrode and an electrode electrically connected in common Control of the light-emitting elements can be provided through an analog data signal line, a select signal line, a power connection and a ground connection. For an example of an active-matrix display having digital driving methods, see U.S. Pat. No. 7,782,311.
One common, prior-art method of forming active-matrix control elements typically deposits thin films of semiconductor materials, such as silicon, onto a glass substrate and then forms the semiconductor materials into transistors and capacitors through photolithographic processes. The thin-film silicon can be either amorphous or polycrystalline. Thin-film transistors (TFTs) made from amorphous or polycrystalline silicon are relatively large and have lower performance compared to conventional transistors made in crystalline silicon wafers. Moreover, such thin-film devices typically exhibit local or large-area non-uniformity across the glass substrate that results in non-uniformity in the electrical performance and visual appearance of displays employing such materials. In such active-matrix designs, each light-emitting element requires a separate connection to a driving circuit.
Both the active-matrix and the passive-matrix control schemes rely on matrix addressing; the use of two control lines for each pixel element to select one or more pixels. This technique is used because other schemes such as direct addressing (for example as used in memory devices) require the use of address decoding circuitry that is very difficult to form on a conventional thin-film active-matrix backplane and impossible to form on a passive-matrix backplane. Another data communication scheme, for example used in CCD image sensors as taught in U.S. Pat. No. 7,078,670, employs a parallel data shift from one row of sensors to another row, and eventually to a serial shift register that is used to output the data from each sensor element. This arrangement requires interconnections between every row of sensors and an additional, high-speed serial shift register. Moreover, the logic required to support such data shifting would require so much space in a conventional thin-film transistor active-matrix backplane that the resolution of the device would be severely limited and is impossible in a passive-matrix backplane.
Active-matrix elements are not necessarily limited to displays and can be distributed over a substrate and employed in other applications requiring spatially distributed control. The same number of external control lines (except for power and ground) can be employed in an active-matrix device as in a passive-matrix device. However, in an active-matrix device, each light-emitting element has a separate driving connection from a control circuit and is active even when not selected for data deposition so that flicker is eliminated.
Employing an alternative control technique, Matsumura et al., in U.S. Patent Application Publication No. 2006/0055864, describe crystalline silicon substrates used for driving LCD displays. The application describes a method for selectively transferring and affixing pixel-control devices made from first semiconductor substrates onto a second planar display substrate. Wiring interconnections within the pixel-control device and connections from busses and control electrodes to the pixel-control device are shown. A matrix-addressing pixel control technique is taught and therefore suffers from the same limitations as noted above.
WO2010046638 describes active matrix devices with chiplets connected in a logical chain.
The signals that actually control the pixel elements in a display area on a flat-panel display substrate are analog, that is the amount of light emitted or controlled by the pixel element is continuously responsive to a continuous signal, for example a voltage or current signal. Image data, for example the originally-broadcast television image signals, can be likewise analog. However, image data is often stored and transmitted in a digital format. In order to display an image, therefore, the digital image data is converted to an analog form and then transmitted to the display, for example as disclosed in U.S. Pat. No. 6,888,523. U.S. Pat. No. 7,259,740 discloses locating a digital-to-analog converter in data-line driver chips that are affixed to the display substrate external to the display area. Unfortunately, the analog data signals are subject to degradation in both transmission and storage, particularly for large displays and for passive-matrix displays. Large transmission line effects in data and control lines on a large display can reduce frame frequency and thereby induce flicker or increase the current and voltage signal line drive requirements beyond what is feasible.
One mechanism for employing digital signals to drive display pixels is to employ time-domain pulse-width modulation, for example as disclosed in U.S. Patent Application No. 2007/0252855. In this technique, each image frame is temporally subdivided into shorter sub-frames that are too short to be distinguishable to a viewer so that flicker is not induced. Pixels are then turned fully on or fully off during the sub-frame times. The proportion of time that a pixel is turned on corresponds to the relative gray level of the pixel. For example, a pixel at maximum brightness is turned on 100% of the time; a pixel at 50% brightness is turned on 50% of the time, and so on. This method, however, requires very high-frequency control signals to provide an adequate gray scale resolution; such high-frequency signals can be difficult to maintain in a flat-panel display, especially a large display having lengthy signal lines. Another method employing digital signals is described in commonly assigned US 2010/0156766.
A related method described in “A Novel Low-Power-Consumption All-Digital System-on-Glass Display with Serial Interface” in the Society for Information Display Digest 2009, 28.1, subdivides a pixel into separate, digitally controlled pixel portions that are each activated to emit an amount of light corresponding to the area of the pixel portion, for example by supplying a common current density to all of the pixel portions, regardless of area. The pixel portions can have different areas, for example varying in area by powers of two. By controlling a desired combination of pixel portions, a gray scale is provided. However, such a method requires separate construction and control of the portions, decreasing the aperture ratio and increasing circuit size.
There is a need, therefore, for an improved control method for display devices that overcomes the control and wiring problems noted above.